net/http: TimeoutHandler is incompatible with a ReverseProxy that has a negative FlushInterval

[hochhaus]

What version of Go are you using (go version)?

1.13.0 fails
1.12.9 works correctly

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/ahochhaus/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="[elided]"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go-1.13"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.13/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build921412516=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run the following example code under both go 1.13 and 1.12.9.

package main

import (
  "log"
  "net/http"
  "net/http/httputil"
  "net/url"
  "time"
)

func main() {
  // setup base http server                                                     
  http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    http.Redirect(w, r, "/redirectURL", http.StatusFound)
  })
  go func() { log.Fatal(http.ListenAndServe("localhost:1024", nil)) }()

  // setup reverse proxy                                                        
  url, err := url.Parse("http://localhost:1024/")
  if err != nil {
    log.Fatal(err)
  }
  rp := httputil.NewSingleHostReverseProxy(url)
  rp.FlushInterval = -1
  rpServ := &http.Server{
    Addr:    "localhost:1025",
    Handler: http.TimeoutHandler(rp, time.Minute, "msg"),
  }
  go func() { log.Fatal(rpServ.ListenAndServe()) }()

  // send request                                                               
  c := &http.Client{
    CheckRedirect: func(req *http.Request, via []*http.Request) error {
      return http.ErrUseLastResponse
    },
  }
  resp, err := c.Get("http://localhost:1025/")
  if err != nil {
    log.Fatal(err)
  }
  if resp.StatusCode != http.StatusFound {
    log.Fatalf("expected %d. found: %d", http.StatusFound,
      resp.StatusCode)
  }
  log.Printf("success: %d", resp.StatusCode)
}

What did you expect to see?

2019/09/09 17:10:11 success: 302

What did you see instead?

2019/09/09 17:10:11 expected 302. found: 200

[agnivade]

@bradfitz